Frequency modulated carrier wave receiver



Jan? 1945- P. F. G. HOLST ETAL 2,367,352

FREQUENCY MODULATED CARRIER WAVE RECEIVER Fi'led Dec. 6, 1941 2 Sheefis-Sheet '1 INVENTORS are flla'l'wooi mwg ATTORNEY Jan. 16,1945. P, F, QHOLST 2,367,352

FREQUENCY MODUL'ATED CARRIER WAVE RECEIVER Filed Dec. 6,1941 2 Sheets-Sheet 2- qnnd'j'l'l'uI' IQ k: vvvv vv )nvnvv U gs r g g;

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INVENTOR$- I? PaaZI'/IYoZsQ'Zare Karin 00? BY o. 7)? W I AI'I'ORNEY Patented Jal1. 16,1945

.FREQUENGY Mo reassess m ner) .CA'RRIER WAVE E ,LIPATENT OFFICE 1R IVER Paul';F.'G. Holst and Loren iR. KirkWood OakIyn Na, .-.J., :assignors to Radio lCorporation of :America, a corporation of Delaware Application'Decem'ber 6, 1941, Serial'No. 421,900

.11 Claims. 601.,1250-20.)

Our {present invention relates to receivers .oi angular velocity-modulated carrier waves, and more .particularlyto.receivers of frequency modulated carrier waves.

One of the main objects .of our-presentinvention is .to provide a frequency modulated .carrier .wave (FM) receiver oi .the superheterodyne type utilizing a .local oscillator operating ,ata sub-multiple frequency .of the local oscillation frequency. actually appliedto themixer, and both local oscillation and.signalenergiesbeing applied to .the mixer from a circuit prior .to the mixer input circuit.

. Another important obj ect of our invention is; to provide in a heterodyne receiverzof angularvelocity-modulated carrier waves, .a local oscillator operating at a frequency equal to-one-half .the difference between the center frequency of the received vwaves and the operating intermediate I frequencyfl. F.) value.

.Another important'object of our invention is. toprovide a. superheterodyne receiver of modulated carrier waves having an automatic freciated with the oscillator tank circuit to control the. frequencythereof.

Yetanotherobjectof the invention is .to pro vide ,a novel method, of, and means zfior, mixing signals to produce. a :hete-rodyne trequency; the

method, involving theinjection ..in-to .the. output ers,;and more especially to provide an'FM receiver oft-reliable construction which is capable of economical manufacture and assembly. 7 The novel featureswhich-webelieve tobe characteristic of our invention are set forth in particularity in the appended claims; the invention itself, however, as toboth its organization and method of operation will "best be understoodby reference to the following description taken connection with the drawings in which we have indicated diagrammatically a'circuit organization whereby our invention may-becarried-into efiect.

In the drawings: "Figure l shows'a schematic "diagram of a receiver embodying-the invention;

'Figure'2-shows the circuit details of the receiver;

Figure 3-is the frequency response characteristic of the'ne'twor'kbetween the radio frequency amplifier'and the-mixer. Referring, now, to the accompanying drawings, wherein 'like're ference characters in the figures designate similar, 'cirouit' elements, in Figure 1 there is shown thegeneralized networks of our circuit of a signal amplifier of local oscillations i whose frequency is an (harmonic of the local oscillator fundamental: frequency. and, the. response characteristic of the; network feedingyithe I mixer from the saidampliiier outnutycir'cui t liavceivingFMwaves strange-of 70.4110 'l 2'.6,mega

cycles (mot), and the operating 1. F. value being,

of the order of 5 me, a local oscillator being pmvided. in the .systemto heterodyne. with said waves, said oscillator having a: frequenc sequel.

to tone-hali the difierence between ins-esteem quencyof-desired EM wayesand the; said 1.. 315';

Still other objects of our invention are. mm

novel receiver. It'will be understood that we are not limited to receiving 'FM waves, as

phase modulated -Waves could be received. Hence, "the generic expressionv "angular velocitymodul'ated' carrier'wave'f is employed herein to include both of ithexa'foresaid forms of modulation. While reception is employed by way of-illustratiomtit will be understood that the invention ispnot limited thereto, since features thereof may be "used in amplitude modulated (A'M) carrier wave, reception. Further, it is to be understood that the-receiver may be employed in frequency ranges other than 70.4 to 72.6 mc., and" thatptheIJF'. value canbe other than e-mc. carrier frequency deviation ratio may be high; say of the, :order .of kilocycles '(*kc.) to e'acnsideofthe center frequency (E0). of course,

I where desirable a higher or lowerpdeviation ratio may the employed.

' rngr igurje v1, the collected signals with the centerfreguen-cy Eaare. applied to a radio frequency amplifier. Afl'ocal, oscillator produces oscillations of'irequency Fe. These oscillations are fed 'to the output circuit of the amplifier. The value of F0 is one-half of Fc-"I. .:F; Between the radio amplifier and the mixer network is employed .a. circuitwhose ,frequency response is double-peaked,

yes. showninFigure I'Peak. 2'Ftfis ofa frequency equal ltoithe second harmonic of F PeaklFe is v prove the efficiency and reliability of FM receiv- 66 the desired center frequency. The peaks are spa'cedby the value of the I. F.

The I. F. output of the mixer is amplified, and srbjected to FM demodulation. The demodulator embodies the usual discriminator section to translate the FM waves into AM waves at the I. F. value. The rectifier derives from the AM waves, modulation voltage and direct current voltage. The former corresponds to the modulation originally used to vary the carrier frequency at the transmitter. The latter is used by the AFC circuit to control the oscillator frequency in a sense to maintain the I. F. value at its predetermined magnitude.

The oscillator is operated at a relat vely low radio frequency, of the order of half the ultimate heterodyning frequency, to facilitate AFC action. A two-stage reactance simulation network is employed for correcting the frequency of the oscillator tank circuit. A capacity effect is provided across the tank circuit by feeding oscillatory energy F to a phase amplifier. The output of the phase amplifier is fed to a capacity amplifier which provides the capacity effect across the tank circuit. The magnitude of the capacity effect. as well as the sense of magnitude change, is controlled by the AFC bias which is applied to the phase amplifier.

In Figure 2 there is shown the specific circuit details of the system schematically re resented in Figure 1. The radio frequency amplifier comprises a tube I whose cathode is grounded, and whose control grid is connected to the high potential side of the input circuit 2. The input circuit comprises a band pass tuned primarytuned secondary transformer. in which both circuits are fixedlytuned and have been adjusted to cover the ultra-high frequency tuning range which has been previously referred to. In other words, the input is a bandpass link circuit covering the entire band. The low potential side of the input circuit may include a series resistor 4 which is by-passed by condenser 4 for carrier currents. Any desired type of signal collector device, or transmission line, may be coupled to the input coil of circuit 2 provided its characacteristic impedance is matched correctly. Such a signal collector device may consist of a dipole, loop, or an ultra-high radio frequency transmission line. In any event, it will be understood that the antenna circuit and input circuit 2 have a frequency response characteristic which is sufficiently wide to pass substantially all the modulated carrier waves of the entire band. The network 4-4 may provide amplitude limiting action to minimize input circuit loading.

The plate of pentode tube I is preferably connected to the positive terminal of the direct current voltage supply source. The connection is made through a radio frequency choke coil 5 whose lower end is by-passed to ground for ra-. dio frequencies by a by-pass condenser. The

.ing of the I. F. transformer I5.

iable condenser II are also-established at an invariable radio frequency potential. The mixer, or first detector, tube I2 may also be of the pentode type, and may be similar to tube l. The cathode of tube I2 is established at ground potential, while the control grid thereof is connected to ground through the grid leak resistor I4. "The plate of the mixer tube is connected to the'proper positive terminal on the direct current voltage supply through the primary wind- The primary resonant circuit I6 is fixedly tuned to the operating I. F. value. The latter, as stated before, is of the order of 5 mo.

As indicated previously, the network 7-6,

-I0II is so constructed and designed that it has the frequency response characteristic shown in Figure 3. It will be observed from that characteristic that the tuned primary and secondary circuits of transformer 9 are sufiiciently overcoupled to provide the double-peaked curve with the deep valley between the peaks. The left peak is located at a frequency which i the second harmonic of F0, the latter being the value of the frequency of the local oscillator. The second peak is located at the center frequency Fe of the selected modulated carrier waves. The peaks are spaced by a frequency value equal to the I. F. Hence, it will be seen that when the receiver is adjusted to receive a desired value of Fe of a given FM channel, then the condensers I, II and 20 are concurrently adjusted so that for any adjustment of the tuning condensers there will be produced FM waves across the I. F. output circuit I5 Whose center frequency is of the operating I. F. value.

Referring now to the local oscillator, it will be seen that that stage comprises a pentode tube H,

which may be the same type of tube as I and I2. The tunable tank circuit is designated by numeral l8, and comprises a coil I9 shunted by variable tuning condenser 29. The high potential side of tank circuit Ill-2c is connected by the grid condenser ZI to the control grid of tube II, the grid leak resistor 22 connecting the control grid to ground. The low potential side of output load impedance of amplifier tube I comprises a parallel resonant circuit consisting of coil 6 and adjustable condenser I. The high potential side of resonant circuit 'I6 "is connected to the plate end of coil 5 by a directcurrent blocking condenser 8. The low potential ends of condenser I and coil 6 are established at an invariable radio frequency potential. The coil 6 is the primary winding of a transformer 9 whose secondary winding I0 is shunted by the variable tuning condenser I I.

The high potential ends of coil I0 and .condenser II are coupled to the input grid of the following mixer tube I2 by the condenser I3, The low potentialends of the coil I l and V3.

tank circuit I8 is established at an invariable radio frequency potential. The cathode of tube II is connected to an appropriate intermediate point on the tank coil I9.

The plate of tube I1 is connected by lead 23 to the plate end of choke coil 5. Hence, it will be seen that both the plates of the radio frequency amplifier tube I and the oscillator tube II are provided with positive voltage from a F-I. F. 'It will be understood that when the tuning condensers! and II are adjustedconcurrently to a given value of F0, then simultaneously the tuning condenser 28 willbe adjusted to that value of F0 which will constantly provide the op-' erating I. F. value. From the circuit shown, it will be appreciated that the local oscillatory energy is injected by lead 23 into the output circuit of the amplifier I. This i quite contrary to the usual practice, wherein. the local oscillations are applied directly to an electrode of the'first detect r tuba.

' frequency.

intros-decided to per-atethelocal-osoillator-at one half the frequency of the energy actually mixed with the signal enereypbeoauseit' -has found 'that the difficultyencountered in controlling the frequency of the localosoillator be comes the greater the-higher the- =local oscillator While the oscillator circuit is gen.-

erally of the *Hartley:type,=it difiersxfrom the conventional Hartley circuit-' in thattthe screen electrode has been by-passed to =ground,lwhile1 inzthe platecircuit of the .oscillator rtube' rant-impedance.

network has been inserted. ,"Thisiimpedance network may generally-she; considered the network e I An action .of':the aforesaid impedance-network be ltwice the frequency/o1 *theoscillatory energy 7 iedzthrough lead 23.: :T-he untuned radio frequency choketcoil. 5, asastatedabeforelsuppliesethe plates of the'oscillatortuhe1- and radio frequency amplifier tube in parallel. Inasmuch asriboth I tubestarerpentodes, .the' loading effect of each one onctherother fist-negligible. Another way oflcone sidlening'lthe impedance providedyim the -.-plate -cin-' cuitnf the oscillator tube H is 'by referringto the "left peak, in the; response characteristic of Eigure;3.e Itmaybestated.a-that the impedancezin "the-plate circuitof the oscillator tubevcorresponds to the peak ZFO in the complete; 'coupli-ngmetwork tion of network 30, is applied to thedemodulator' The-anode-39 of the second diode-of tube 36 is ctmnectedto theung-rounded end of resistor 35,

hil h c h f-that diodeis established at a o n -potential.

h r ar ir0l it3| is overcoupled tor t secondary which consists of 33-34 and 34'. The

impedance characteristic of the network will, therefore, be adouble peaked response curve. with .a-dipatthe 5'mc. I. Evalue. Circuit 33-resonates Circuit-33 and 34' inseries resonate resonateat' 5 Inc. The alternating voltage across 33 .andi34 in series'is rectified by'diode 39 thereby Causing negative direct. current. voltage to appear across resistor 35. Likewise, the voltage across ,34' is. rectified. by diode,3l and the resulta l-t Po itive voltage appearsacross. resistor 38. Hence, if the ,I. F; energy mid-band frequency is at,1nc,, the voltagesacross 35 and :38 are mediurnand the AFC biasiszero. If themid-band frequency is below5 mc., the voltage across 38 is high and the AFC bias is positive. Ifthemidband fr qu ncyis a ove mc-rthe ne a ve .v ta eacr ss r s to fi predo na s and th AFC bias is ne at v The discriminatQr-rectifier is of the singleend'ed type. The modulation voltage is taken 0.1T from the cathode end of resistor '38, and may be used any subsequent audio utilization network. TheAEC bias is also derived from the cathodeend of resistor 38. It is desirable, for a reason to'be' explained at a later point; that the AFC bias have anegative polarity when the mid-band frequency of their F. energy developed across vcircuit; 3.! is higher than the predetermined operating I. F.

value.

network; The demodulator, which-may be of any 2 -m aybe'of the general WP? disclosed" and cla med bri .-v A- G een in U. .S IBa ent .2;.2.8;229 rautedlu y c 9 1;- The e als maybe used deinpd latu networks qf'fl jle true shownhyi-Iahnll flel fd-, nann i atiq SSi T E l Q- 3. 19 3.( .1i i A gus when More specifically, we prefer to utilize a deniodl;

ulet nn wa t a ,t etuneshown int e a ores d Reid anul ati a- 3 .1.e num ra en tes the a res n nt prima ecir u t: nr sedin theoutnu at the last-1 amnlifie firmsi ir u t is tu e toe-the operating I. .ualue, and :has kin shunt therewith are istor 2 toprovide-nroper dampin of the circuit. ll'he secondary-ci cui is desine ed :by numeral endtnehi p tent a ide Qfis connected teth high tentia sne r circuit 7 the couplin condenser lo oten ia side of circuit {is connected t zsraundzthmush QQXIQQMBLM. time capacitor.-

There will now be considered the manner in which frequency correction is applied to the oscillatortank circuit [8 A portion of thelocal oscillatqr n rgy is taken off from-the cathode tap on tank coil l9, and theenergy is impressed upon the controlgrid ofapentode 40. The oscillatory'en- 'ergyistransmitted to the control grid 4| through a pathincluding the couplingncondenser 42. The

,tube 40 functions as a phase amplifier, and there is also applied to its grid 4| the AFC bias. The cathpde of tube is connected to ground through a grid biasing network 43, and the plateof tube 40 is connected to a point of proper positive potential on the direct current voltage supply through a plate resistor' .44. The lower end of resistor 44 is connected by resistor 45 to the oathod'e-endof'thebiasing-resistor 43. The AFC lead is designated by numeral 46, and includes-a filter choke coil 52. The plate end pf coil 52 is connected to the high potential side of tank circuit 18 by the coupling condenser 53. Hence, it will be seen that the plate to cathode impedance of tube 50 is connected across the tank circuit I8. This tube impedance simulates a capacitative reactance, and, hence, the expression capacity amplifier is used in connection with the function of tube 50. Th tube 50 actually functions to amplify a capacitative reactance effect applied to the input electrodes of the tube. The control grid of tube 50 is denoted by numeral 54, and the latter is connected to ground through the grid leak resistor 55. The grid side of resistor 55 is connected to the plate end of resistor 44 by the direct current blocking condenser 56. The numeral 51 designates the capacity which is the principal plate load in tube 40.

In considering th action of the frequency correction network, it is first pointed out that by virtue of a low plus B voltage employed on the plate of the oscillator tube together with the relatively high oscillator frequency, there is required a two tube circuit in the correction network in order to obtain sufficient reactance control. A fraction of the oscillator voltage existing between the oscillator coil tap and ground is obtained by means of a capacity bleeder and is fed to the con trol grid 54 of the fixed-bias phase amplifier tube 40. The resistor 45 is used to provide semifixed bias in tube 40. The plate current in tube 40 will be in phase with the input voltage, but inasmuch as the load in the plate circuit (due to tube and distributed capacitance) is almost pure capacitance, it follows that the voltage impressed on the grid 54 of tube 50 will be 90 degrees ahead of the grid voltage in tube 40,. Therefore, the plate current drawn by the capacity amplifier tube 50 will be 90 degrees ahead of the alternating current plate voltage of the same tube, and this voltage-to-current relation denotes a capacitative load on the tank circuit l8.

If the gain of the phase amplifier tube 40 is varied by adjustment of the control grid bias, it will be seen that the effective capacitance load of tube 50 will vary. More specifically, if the bias applied to grid 4| is made more negative, the capacitative load of tube 50 will decrease, and the local oscillator frequency will increase. Since the oscillator frequency is lower than the signal frequency, an increase in oscillator frequency will result in a decrease of the operating I. F. value. It is, therefore, necessary that the demodulator circuit be designed in a manner which will cause I. F. energy to produce a negative AFC bias when its mid-band frequency is higher than the operating I. F. value. In prior circuits of the singleended discriminator type a positive AFC bias is usually produced when the mid-band frequency of the I. F. energy is higher than the operating I. F. value. Hence, the discriminator-rectifier sections of the present demodulator network have been designed in the present circuit so as to provide a negative AFC bias when the mid-band frequency of the I. F. energy is higher than the operating I. F. value.

It has been found that the AFC circuit disclosed herein is of advantage in a system wherein the voltage regulation on the oscillator tube is poor. The AFC circuit functions in a highly satisfactory manner and regulates the tank frequency in such a-manner that th operating I. F. value is maintained substantially constant. The principal objects which made-it desirable to add an AFC circuit to this receiver are in order of their importance:.

(1) Ease of tuning.

(2) Overcome the effects of transmitter frequency drift,

(3) Overcome the effects of receiver oscillator frequency drift.

If the power supply source is a storage battery source which gradually becomes weaker after the passage of time, the AFC acts to overcome oscillator frequency drift. The power supply source may be the usual power rectifier for operation with a commercial alternating current source. It is, also, pointed out that the present AFC system is of particular advantage in the case of a mobile receiver unit which is encased in a housing which does not permit ready change of any battery source. In such case the AFC circuit must function'to'maintain a high degree of correction in spite of considerable variation of the direct current voltage supply.

While we have indicated and described a system for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing fromthe scope of our invention. as set forth in the appended claims. I

What we claim is:

1. In a modulated carrier wave receiving system'of the heterodyne type, a mixer stage provided with an output circuit tuned to a desired heterodyne frequency, a signal amplifier stage, a local oscillator stage tuned to a frequency which is equal to one half the difference between the desired signal frequency and said heterodyne frequency, a reasonant network having a double peaked response curve coupling the output electrodes of said signal amplifier stage and the mixer input electrodes, means for injecting the oscillatory output of said oscillator stage into the input of said coupling'network, and said coupling network response curve having one peak at double the frequency of the oscillations produced by said oscillator stage and the second peak at the signal carrier frequency.

2. In a modulated carrier wave receiving system of the heterodyne type, a mixer stage provided with an output circuit tuned to a desired heterodyne frequency, a signal amplifier stage, a local oscillator stage tuned to a frequency which is equal to one half the difference between the desired signal frequency and said heterodyne frequency, a resonant network coupling the output electrodes of said signal amplifier stage and the mixer input electrodes, means for injecting the oscillatory output of said oscillator stage into the input of said coupling network, and said coupling network having a frequency response characteristic with spaced peak frequencies at values equal to double oscillator frequency and the desired signal frequency.

3. In a modulated carrier wave receiving sys-, tem of the heterodyne type, a mixer stage provided with an output circuit tuned to a desired heterodyne frequency, a signal amplifier stage, a local oscillator stage tuned to a frequency which is equal to substantially one half the difference between the desired signal frequency and said heterodyne frequency, a resonant network coupling the output electrodes of said signal amplifier stage and the mixer input electrodes, means for injecting the oscillatory output of said oscillator stage into the said coupling network,

the frequency of the oscillations produced by said .oscillatorstage, and means, responsive toga: shift of the heterodyne energy from saidapr'edeter mined heterodyne frequency, for adjusting said oscillator stage frequency in a sense to maintain said heterodyne energy at said desired heterodyne frequency and said oscillator stage frequencytbe ing of a sufficiently low radio frequencytofacileita-te' said oscillator frequency adjustingaction.

4. Ina modulated carrier wave receiving system of the heterodyne type,'a mixer stage provid ed with an output circuit tuned to a;desired=,heterodyne frequencypa signal amplifier stage, a local oscillator stage tu ned to a frequency which is equal to one half the differencebetween the desired signal frequency and said heterodyne frequency, a resonant network coupling the output electrodes of said signal amplifier stage and the mixer input electrodes, means for injecting the oscillatory output of said oscillator stage into the input of said coupling network, said coupling network having an output circuit whose impedance is such as to double the frequency'of the oscillations produced by said oscillator stage, said coupling network consisting of an overcoupled pair of tuned circuits having a double peaked response curve whosepeak frequencies are spaced by the Valueof said heterodyne frequency, said signal amplifier stage having a source of angular velocity-modulated carrier waves coupled to its input electrodes, and a heterodyne amplifiernetworkcoupled to the mixer output circuit, said heterodyne amplifier network additionally functioning to minimize amplitude modulation effects on the received carrier waves.

; In a frequency modulated carrier'wave receiving system of the heterodyne type, a converter provided with an output circuittuned to a desired heterodyne frequency, a signal amplifier stage, a local oscillator stage tuned to a frequency which is equal to one half the difference between the desired signal frequency and said heterodyne frequency, a resonant network coupling the output electrodes of said signal amplifier stage and the converter input electrodes, means for applying the oscillatory output of said oscillator stage to said coupling network, said coupling network being constructed and arranged to double the frequency of the-oscillations produced by'said oscillator stage and said coupling network consisting of a pair of tuned circuits overcoupled to provide a double peaked response curve whosepeakfrequencies are separated by the value of the heterodyne frequency.

.6. In a frequency modulated carrier wave receiving system of the superheterodyne type, a first detector stage provided with an output circuit tuned to a desired intermediate frequency, a signal amplifier stage, a local oscillator stage tuned to a relatively low radio frequency, a resonant network coupling the output electrodes of said signal amplifier stage and the first detector input I the input terminals of said coupling network, saidstage provided withan outputoircuit tuned to a desiredheterodyne frequency, asignal amplifier stage,-.a-loca1- oscillator stage tuned to a frequency which is equalto --one half the difierence between-thedesired signal frequency and said heterody-ne. frequency, a resonant'network couplingthe output electrodes of said signal amplifier stage and the mixer input electrodes, means for injecting the oscillatory output of said oscillator stage into the input of said coupling network, said coupling network having an output circuit whose impedance issuch as, todouble the frequency of the oscillations producedby said oscillator stage, means, responsive toa shiftof the heterodyne energy from said predetermi;n edheterodyne frequency, for adjusting said oscillator stage frequency in a sense to maintain said heterodyne energy at said desired heterodyne frequency, said last means including a pair of tubes functioning to provide a capacitative reactance across the oscillator and said oscillator stage frequency being of a sufficiently low radio frequency to facilitate said oscillator frequency adjusting action.

8. In a modulated carrier wave receiving system of the heterodynetype, a mixer stage provided with an output circuit tuned to a desired heterodyne frequency, a signal amplifier, stage, a local oscillator stage tuned to a frequency which is equal to one half the difference between the desired signal frequency and said heterodyne frequency, a resonant network coupling the output electrodes of said signal amplifier stage and the mixer input electrodes, means for injecting the oscillatory output of said oscillator stage into the input of said coupling network, said coupling net- .work being constructed andarranged to provide a double peaked response curve having itsspaced peak frequencies separated by said heterodyne modulation effects on the received carrier waves.

9. In a modulated carrier wave receiving system. of the heterodynetype, a mixer stage provided with an output circuit tuned to a desired heterodyne frequency, a signal input network, a local oscillator stage tuned to a radio frequency which is relatively lower than the signal frequency, a resonant network having input and output terminals coupling the signal input network and the mixer input electrodes, means for injecting the oscillatory output of said oscillator stage into coupling network having an impedance such as to double the frequency of the oscillations produced by said oscillator stage and said coupling network consisting of a pair of overcoupled tuned circuits whose response curve is double peaked with spaced peak frequencies at the double frequency and signal frequency respectively.

10. In a signal carrier Wave receiving system of the heterodyne type, a mixer stage provided with an output circuit tuned to a desired heterodyne frequency, a signal input source, a local oscillator stage tuned to afrenquency which is equalto one half the difference between the desired signal carrier frequency and said heterodyne frequency, a resonant network coupling said signal source and the mixer input electrodes, means for injecting the oscillatory output of said oscildouble the frequency of the oscillations produced by said oscillator stage, and said coupling network having a double peaked frequency response characteristic with spaced peak frequencies at values respectively equal to said double oscillator frequency and the desired signal carrier frequency.

11. In a modulated signal carrier wave receiving system of the heterodyne type, a mixer stage provided with an output circuit tuned to a desired heterodyne frequency, a signal input stage, a local oscillator stage tuned to a frequency which is equal to one half the difference between a desired signal carrier frequency and said heterodyne frequency, a resonant network coupling the said signal output stage and the mixer input electrodes, means for injecting the oscillatory output of said oscillator stage into said coupling network, said coupling network having an' impedance such as to double the frequency of the oscillations produced by said oscillator stage, and means, responsive to a shift of the heterodyne energy from said predetermined heterodyne frequency, for adlusting said oscillator stage frequency in a sense to maintain said heterodyne energy at said desired heterodyne frequency and said oscillator stage frequency being of a sufiiciently low radio frequency to facilitate said oscillator frequency adjusting action.

PAUL F: G. HOLST.

LOREN R. KIRKWOOD. 

